It is often desirable to obtain measurements of selected characteristics of sheet materials during manufacture. Although various properties of sheet materials can be detected by off-line laboratory testing, this procedure is often not practical because of the time required for sample acquisition and analysis. Also, laboratory testing has the shortcoming that samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the drawbacks of laboratory testing of sheet materials, various sensor systems have been developed for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Typically, on-line sensor devices are operated to periodically traverse, or “scan,” traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about twelve meters or more.
Terahertz systems known as terahertz time-domain spectrometers (THz-TDS) often use laser pulses each lasting only 10 to 200 femtoseconds to generate, detect, and measure electromagnetic pulses (“T-rays”) that each last for about a picosecond. T-rays can be transmitted through various objects, using an imaging system of lenses and mirrors to focus or collimate the T-rays. As the T-rays pass through the object under test, they are typically distorted. These changes in the T-ray signals can be analyzed to determine properties of the object. Materials can be characterized by measuring the amounts of distortion-from absorption, dispersion and reflection-of the T-rays passing through to a detector. A digital signal processing unit processes the data and translates it into data that appear on a computer screen. The digital signal processor takes the digitized data from the THz detector and analyzes the data in either the spectral or temporal domain.
Because many compounds change T-rays in characteristic ways (e.g., absorption or dispersion), molecules and chemical compounds, show strong absorption lines that can serve as “fingerprints” of the molecules. T-ray spectroscopy can distinguish between different chemical compositions inside a material even when the object looks uniform in visible light. Typical THz-TDS devices are designed for batch applications and therefore are not suitable for deployment for on-line applications.